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Abstract :
[en] The Southern Ocean is a hotspot for gas exchange, accounting for ~40% of the global ocean CO2 sink thanks to the biological pump. Despite advances in satellite and direct measurements, large gaps remain in our understanding of the in-situ drivers of carbon export, particularly plankton community composition. During the Antarctic Circumpolar Expedition (ACE) in austral summer 2016/2017, we investigated the composition, distribution, and trophic structure of plankton communities across the different hydrographic zones of the Southern Ocean through measurements of the carbon and nitrogen isotope ratios of surface suspended particulate matter (δ13CSPM and δ15NSPM; 332 stations) and various mesozooplankton species (28 stations). Our approach involved: 1) Using a two-endmember isotope mixing model to determine nitrate- vs. recycled ammonium-supported phytoplankton growth, and 2) assessing the role of mesozooplankton in carbon export. We found that 43 ± 28% of photoautotrophic carbon was potentially exportable during the growing season, with the highest export potential estimated for the southern Boundary Zone and near the (Sub)Antarctic islands (50-98%). Along with iron, phytoplankton community composition emerged as an important driver of the biological pump, with large diatoms dominating regions characterized by high nitrate dependence and elevated carbon export potential and smaller, mainly non-diatom taxa proliferating in waters where recycled ammonium supported most productivity. The isotopes of the mesozooplankton revealed contrasting trophic behaviours between Subantarctic and Antarctic populations. Herbivorous behaviour in the Subantarctic likely promotes in-situ carbon recycling, while the more omnivorous/carnivorous zooplankton that dominate the Antarctic may facilitate greater carbon export. This study highlights the utility of large-scale isoscapes for investigating the ocean’s biological pump.
